Why are my feet sometimes purple?

Are you a diabetic? If your not sure you might want to go get checked out. Purpleing of hands and feet can be a sign of lower blood circulation, and if your diabetic this can lead to amputation of hands, arms, feet or legs. I'm not saying that's the problem, but you really should see a doctor, they will run some test.

Yang deficiency of the spleen and kidney and invasion of pathogenic cold. it happens especially when u are excited emotionally. then it turns red and then normal.

It's probably Raynaud's http://en.wikipedia.org/wiki/Raynaud's_phenomenon See a doctor to find the reason for it.